Integration — practice question

The diagram depicts triangle $OAB$, where angle $ABO$ is a right angle, angle $AOB = \frac{\pi}{5}$ radians, and $AB = 5\text{ cm}$. Arc $BC$ is part of a circle centred at $A$ and intersects $OA$ at $C$. Arc $CD$ is part of a circle centred at $O$ and intersects $OB$ at $D$.